The invention relates generally to code-division-multiple-access (CDMA) networks, and more particularly to detecting multiple signals in a channel subject to multi-access interference, multi-path fading, and varying power levels of transmitters.
There is a significant interest in designing code-division multiple-access (CDMA) spread spectrum networks that allow data rates on the order of ten""s of megabits per second, or even higher asynchronous rates. For example, there is a demand for very high speed wireless networks to support multi-media applications. In a code-division multiple-access (CDMA) network, multiple transmitters share a common channel by modulating a set of signature waveforms.
Some of these networks are based on an interim standard, see xe2x80x9cEIA/TIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d Telecommunications Industry Association, July 1993. For digital signals, direct sequence (DS) coding is typically used to achieve spread spectrum coding.
In DS coding, the data (symbols) are mixed with a very high rate coding signal. Typically, the DS coding signal is generated by a pseudo-random code generator. The amount of spectrum spreading achieved is simply the ratio of the code rate to the data rate. The code rate is also known as the xe2x80x9cchip rate.xe2x80x9d Twice the chip rate doubles the occupied spectrum of the DS transmitted signal. The duration of a chip sequence Tc is related to the symbol interval T by Tc=T/N.
In a CDMA network, receivers observe the sum of the transmitted signals, plus any noise. Multiple-access deals with detecting mutually interfering digital signals. Sometimes, the superposition of the signals sent by different transmitters occurs unintentionally due to non-linear effects, such as, cross-talk in telephony and multi-track magnetic recording, and when the same radio frequency band is used simultaneously by multiple transmitters as in cellular telephony, personal communications services (PCS), digital television (DTV) broadcasting, and wireless local loops (WLL).
Many wireless systems operate under highly dynamic conditions due to the mobility of the transmitters, varying environmental conditions, and the random nature of channel access. Detecting signals at a base station of a wireless system, encounters many difficulties. For example, high speed CDMA networks suffer from multi-access interference, multi-paths and echoes.
In terrestrial Digital TV (DTV) transmission with a single transmitter sending symbols at a rate of 10.76 Mbits per second, echoes with delays of more than thirty microseconds can interfere with the main signal. This is referred to as a fading channel. This means that equalizers with several hundreds of taps are required. In the multi-user scenario, the base station has to detect the data streams of multiple transmitters from the combined received data stream. This is referred to as multi-access interference between different transmitters (or users).
This interference problem assumes more serious proportions in cellular networks. Due to the mobility of the transmitters, signal strength varies. The strength of the signal from a transmitter closer to a base station is stronger than the signal from a transmitter further away. The signals from the closer transmitter can completely overpower the weaker signal. This is the so-called near-far problem.
The near-far problem can be avoided by power control. The basic idea of power control is to provide feedback to mobile transmitters to control their transmitted power levels to yield equal power at the receiver from-all mobile transmitters. Some work has focussed on looking at the signal strengths of the different transmitters. Stronger transmitters make their decisions first, and these decisions can be used in a successive cancellation method.
Prior art multiple-access detector methods are computationally intensive. As the number of transmitters increases, the computational complexity of receivers will increase exponentially. This renders these methods impractical for implementation in a base station which is expected to concurrently serve tens of transmitters.
Wang et al. in xe2x80x9cBlind Equalization and Multi-user Detection in Dispersive CDMA Channels,xe2x80x9d IEEE Transaction Communications, Vol.46, No. 1, 1998, deals with multi-access interference mitigation. They describe the similarity in a mathematical framework between channel equalization and multi-user detection. Their model for multi-user detection in dispersive CDMA channels uses a least squares method and adaptive signal processing.
U.S. patent application Ser. No. 09/262,377, entitled xe2x80x9cMethod and Apparatus for Equalizing a Digital Signal Received via Multiple Transmission Pathsxe2x80x9d filed by Bao et al. on Mar. 4, 1999 describe a decision feedback equalizer (DFE) that exploits the sparse channel characteristics of terrestrial digital television transmission. A least mean square (LMS) method is used for updating equalizer tap coefficients. Only taps with substantial energy and their neighboring taps are updated.
It is desired to provide a method and apparatus for detecting individual transmitted signals in a CDMA network channel that is subject to multi-access interference between users with varying power levels, multi-path fading and echoes.
The invention provides a method and apparatus for receiving signals in a network configured with code division multiple-access (CDMA ) channels. The receiver according to the invention improves performance in very high speed wireless channels used for multimedia applications such as digital video.
Multipaths and echoes with very long delays are cancelled at high data rates. Signals received from multiple transmitters are detected even when the signals have varying signal strengths. The method according to the invention exhibits superior performance and provides robustness in noisy channels. The method also reduces the number of detector taps that need to be updated.
Examples of multi-access applications which can use the invention include mobile telephony, satellite communications, multidrop telephone lines, local area networks, packet-radio networks, interactive cable television networks, and fixed wireless local loops.
More particularly, the invention provides a receiver in a code-division-multiple-access network. In general, the receiver includes a forward detector, K feedback detectors, where K is the number of transmitters, a slicer and an arithmetic logic unit for each one of multiple transmitted signals. The forward detector has an input connected to receive the combination of the multiple transmitted signals and noise. The arithmetic logic has an input connected to an output of the forward detector.
The slicer has an input connected to an output of the arithmetic logic unit. A first feedback detector has an input connected to an output of the slicer and an output is connected to the input of the arithmetic logic unit. Each of the other Kxe2x88x921 feedback detectors has an input connected to the output of the slicers for each of the other slicers and an output is connected to the input of the arithmetic logic unit. The output of the slicer provides the detected signal for each transmitter.